A plurality of substrate processing apparatuses for polishing and cleaning wafers are installed in a manufacturing factory for semiconductor devices. In the manufacturing factory in recent year, a central supply system is employed for distributing a processing liquid, such as a polishing liquid or a chemical liquid, to the substrate processing apparatuses. In this central supply system, the processing liquid circulates through a circulation line installed in the manufacturing factory, and supplies the processing liquid to the substrate processing apparatuses through branch lines extending from the circulation line.
The processing liquid that has been supplied to the substrate processing apparatus continues to flow in the substrate processing apparatus as long as a wafer is being processed (i.e., polished and cleaned). However, when processing of a wafer is not performed, the processing liquid stays in a pipe installed in the substrate processing apparatus. If processing of a wafer is not performed for a long time, the processing liquid sinks in the pipe. As a result, a concentration distribution of the processing liquid may change, or particles, such as abrasive grains, contained in the processing liquid may condense into large particles (which will be hereinafter referred to as coarse particles). The processing liquid, whose concentration distribution has changed, may cause an adverse influence on a substrate processing result. Moreover, if the coarse particles are brought into contact with a wafer during processing of the wafer, scratches may be caused on a surface of the wafer.